An unidentified suicide bomber blew himself up at a traffic-clogged intersection in the historic district of Midan.

The bomber's target remains unclear, although a police bus appeared to have borne the brunt of the blast. More than 46 people were wounded.

Despite the apparent target, officials said that the vast majority of the casualties were civilians.

"He detonated himself with the aim of killing the largest number of people," said Mohammed Shaar, the interior minister.

The attack came exactly a fortnight after two booby-trapped cars, allegedly driven by suicide bombers, exploded in front of government intelligence buildings in Damascus killing 44 people.

There have been no claims of responsibility for either attack.

Districts in the centre of the capital like Midan have largely been spared the bloodletting that has accompanied the 10-month uprising against Bashar al-Assad, the Syrian president, but the recent bombings are a sign that even those who have not become involved are now vulnerable.

The regime was quick to blame yesterday's attack on "terrorists".

It has used the term to describe many of those involved in the protests in order to justify the intensity of its repression of the uprising, during which more than 5,000 people have been killed, according to UN estimates.

As the death toll has increased, an armed insurgency has emerged, with military defectors forming the rebel Free Syrian Army.

"This is planned and systematic state terrorism by the security forces of President Bashar al-Assad," Major Maher al-Naimi, the group's spokesman said.

The attack coincided with mass protests called to demand Mr Assad's overthrow.